Sealant compositions having, as an essential component, hydrogenated polybutadiene as network polymer

ABSTRACT

A conventional sealant composition having as its essential rubber components (1) at least one high molecular weight, hydrogenated polybutadiene polymer, and (2) at least one low molecular weight liquid elastomer, i.e., polybutene, as tackifying polymer, compatible to a high degree with said hydrogenated polymer; the resultant sealant composition reflecting a co-continuous phase morphology wherein said hydrogenated polymer provides a crosslinked network in said compatible low molecular weight liquid elastomer.

BACKGROUND OF THE INVENTION

For safety, pneumatic tires have been historically sought which have ameans of retarding or preventing their deflation upon being punctured.Many methods and tire constructions have been suggested and used forthis purpose, mostly without significant commercial success forconventional passenger vehicles such as automobiles which are to bedriven over typical roadways; this lack of commercial success has beendue to high cost, complexity as to design and poor sealant performance.Fluid puncture sealants which seal by flowing into the puncture holehave not been entirely successful because they tend to cause the tire tobecome out of balance or tend to have varying flow properties over awide temperature range. Central cores of cellular material which willphysically maintain the tire shape when punctured have been suggested.However, such cores place a serious restriction on the vehicular maximumspeed or endurance due to degradation of the core.

Puncture sealing tubeless tires have previously been proposed,containing, in the area of the tire normally most subject to punctures(that is, the undertread or the area extending across the crown of thetire at least from one shoulder to the other), a layer of sealantcomposition which has viscoelastic and adhesive qualities such that thecomposition tends to stick to a puncturing object, and, when thepuncturing object is withdrawn, tends to flow into the opening orpuncture, forming a plug which seals the opening against loss of airfrom the tires. Unfortunately, it has proven difficult to provide acomposition which would flow into the puncture hole and yet havesufficient viscosity to prevent it from flowing at elevated temperaturessuch as exist in an automobile pneumatic tire under operatingconditions. The problem is complicated by the extreme centrifugal forceto which the composition is subjected as the tire rotates at high speed,since such centrifugal force tends to cause the composition to flow intothe central crown area, leaving the areas near the shoulder unprotected.Furthermore, it has proven difficult to provide a sealant compositionwhich would retain this desired balance of viscosity, adhesion andconformability over an extended period of service.

Various elastomer-based compositions, both cured and uncured, have beenproposed as puncture sealants. In the uncured state, although they mayfunction as sealants, they will sometimes tend to "cold flow" or flow atelevated temperatures such as are encountered in tires during use. Thisflow is undesirable. When they are crosslinked (cured) to prevent flow,these materials can lose the adhesion and conformability of the uncuredstate, and thus no longer act as sealants.

DESCRIPTION OF THE PRIOR ART

Typical prior art patents include the following; the substance of eachis incorporated by reference:

U.S. Pat. No. 3,952,787:

This patent teaches the preparation of a rubber (sealant) compositioncomprising a polymer of ethylene-propylene copolymer orethylene-propylene-diene terpolymer, with polybutene and an inorganicfiller. There is no suggestion or teaching in this patent of ourspecific sealant components or their collective performance as hereindescribed.

U.S. Pat. No. 3,981,342:

This patent is directed to a puncture sealing composition in combinationwith a pneumatic tire; said composition is exemplified by a blend ofdepolymerized NR and 40 parts cis-polyisoprene, partially crosslinkedwith 6 parts of tetra-n-butyl titanate to provide a gel content of about40%. This patent does not teach or suggest our sealant components ortheir collective performance illustrated herein.

U.S. Pat. No. 3,903,947:

This patent teaches the preparation of a laminated puncture sealingstrip for pneumatic tires. The sealant composition contained in saidstrip can be EPDM plasticized with a paraffinic oil, together withconventional additives. There is no disclosure or suggestion in thispatent that would lead one skilled in the art to prepare our uniquesealant rubber blend containing conventional additives.

U.S. Pat. No. 2,756,801:

This patent is directed to the preparation of a puncture-sealingpneumatic tire; the sealant composition comprises butyl rubber modifiedby reaction with a small amount of a phenol such as dimethylol phenol;conventional additives are also utilized. There is no suggestion in thispatent of the use of any hydrogenated polymer per se or in combinationwith a compatible tackifying polymer.

U.S. Pat. No. 2,935,109:

This patent is directed to the preparation of heat-sealable linings foruse, primarily, in tubes and tubeless tires. The rubber component is ablend of soft gum rubber and a minor amount of an ethylene polymer,together with conventional additives. This patent does not disclose orteach any pertinent compounding suggestive of our sealant composition.

U.S. Pat. No. 3,048,509:

This patent teaches the preparation of puncture sealing means forpneumatic tires wherein a multi-layered tacky sealant layer is preparedcontaining conventional additives, the rubber component of said layerbeing a blend of natural rubber and selected SBR rubbers with or withoutreinforcing pigment. Clearly this is not suggestive of our uniquesealant composition with its novel rubber component characterization.

U.S. Pat. No. 3,935,893:

This patent is directed to a sealant layer for a pneumatic tire that ismade up of the combination of specific quantities of a high molecularweight curable butyl rubber, a low molecular weight curable butylrubber, a liquid polybutene tackifier, and a partially hydrogenatedblock copolymer of styrene and a conjugated diene, carbon black, andsuitable curing agents for the butyl rubber components.

U.S. Pat. No. 4,113,799:

This patent relates to carbon reinforced elastomeric sealantcompositions comprising crosslinked butyl rubber as matrix sealant, thesealant composition comprising a high average molecular weight butylrubber and a low average molecular weight butyl rubber in a specifiedratio in admixture with a tackifier with or without including apartially hydrogenated block copolymer.

U.S. Pat. No. 4,064,922:

This patent is directed to a puncture sealing composition and a tire andcombination with such a composition, the sealant comprising essentially:(a) a major proportion of a low molecular weight elastomer in admixturewith a tackifying or plasticizing substance and (b) a minor proportionof a high molecular weight elastomer together with a crosslinking agentpresent in an amount sufficient to give a partial cure.

It is clear that a sealant composition, such as a tire sealantcomposition, in order to be most effective, must meet a set of verystringent requirements. For example, the sealant must not only exhibitthe ability of adhering to a puncturing object such as a nail as thelatter precesses about its initial position during tire travel but mustalso be capable of healing itself so as to effectively seal the puncturesite once the nail is removed. In addition, the sealant must possesssufficient strength and integrity; first, in order to maintain its formstability in the tire; and secondly, to avoid "blow-through" when thenail is removed. The tire sealant composition must be capable ofretaining effectiveness over a wide temperature range (-30 T °C. 130)for an extended period corresponding to the service life of the tire.

The preceding prior art references, when taken singly or in anyreasonable combination thereof, would not teach or suggest our uniquesealant composition or its performance.

THE INVENTION

Our invention is directed to a sealant composition having as anessential component, at least one hydrogenated polybutadiene polymerthat functions in said composition to provide continuity and strength;there is achieved a high degree of mutual compatibility of said polymerand the selected polymeric tackifying component. Our unique compositionpossesses a specific proportion of tackiness, resiliency and strengthwhich renders said composition particularly suitable, for example, foruse as a sealant in pneumatic tires.

Whereas, in practice, tire sealants have traditionally relied heavily onthe use of such polymers as butyl, EPDM, depolymerized rubber, etc., thesealant composition of the present invention is a new composition ofmatter which is based on a polymer blend system employing a highmolecular weight, hydrogenated polybutadiene as network forming polymerin admixture with a low molecular weight liquid elastomer, for example,polybutene, as tackifier in which a high degree of mutual compatibilityexists between the high and low molecular weight polymers.

Our composition contains a network polymer, a tackifier and a range ofvarious ingredients, such as carbon black curatives, etc., normally usedin the formulation of, for example, tire sealant compositions. Thepresent invention is one wherein either all hydrogenated butadiene or ablend of hydrogenated butadiene and another high molecular weightpolymer fully compatible with the particular hyrogenated butadieneselected is used, for example, EPR or EPDM. The tackifier, in turn, canbe either a low molecular weight analogue of the hydrogenatedpolybutadiene network polymer or a low molecular weight analogue of theother high molecular weight network polymer heretofore referred to. Thetackifier, however, can be a low molecular weight polymer of an entirelydifferent type, for example, polybutene, the proviso being that it befully compatible with the selected network polymer(s).

The hydrogenated polybutadiene should be selected from those havingbetween 10 and 98% vinyl content prior to hydrogenation and a level ofhydrogenation between 40 and 98%; the molecular weight of this networkpolymer should be between 80,000 and 400,000 for an acceptable degree ofcompatibility and sealant performance. We have shown that commercialbutadiene (not hydrogenated) is grossly incompatible with polybutene atthe levels required of a tire sealant composition.

The present invention requires a precise balance among tackiness,resiliency and strength which renders the composition effectiveness as asealant for use under dynamic conditions over a wide temperature rangefor extended periods of time.

The sealant composition of the present invention possesses thecapability of providing, via various modes, an exacting balance betweentackiness, resiliency and strength which are essential to effectivesealant performance. Said sealant composition is capable of being"converted" to the sealant state either by conventional chemical curingmeans, by electron beam irradiation treatment, or by a combination ofthese methods. To be effective, our sealant composition is prepared topossess a unique balance as to tackiness, resiliency and strength; weutilize, as herein described, a high molecular weight, network-formingpolymer in combination with a low molecular weight, tackifying polymerwherein the mutual compatibility of said polymers is of a high degree.

We have discovered that sealant performance can be significantly alteredby changes in the rheology of our sealant composition to insure optimumsealant performance. We employ a network polymer whose rheology can, notonly be varied by design, but controlled as well. The use ofhydrogenated polybutadiene offers such rheological control in that itsmolecular weight, molecular weight distribution and its microstructurecan be designed and controlled within narrow limits to meet sealantrequirements.

A significant feature of our discovery is that the latitude inrheological properties of the composition can be extended by blendingtwo or more hydrogenated polybutadiene polymers of different individualrheologies. For example, a hydrogenated, high vinyl polymer, whenblended with a hydrogenated, low vinyl polymer, could particularly besuited for use in a sealant composition; this is opposed to the"conventional" polybutadiene since the hydrogenated structure results inlower residual unsaturation to offer greater heat stability. The keyadvantage to the use of hydrogenated polybutadiene in the practice ofour invention is the dual capability of polymer design flexibility andgreater heat stability--a combination which is difficult to attain withother elastomers.

The preferred number average molecular weight range for the hydrogenatedpolybutadiene that can be utilized in the practice of our invention isfrom 80,000 to 400,000; although an hydrogenated polybutadiene with anumber average molecular weight outside this range would provide acomposition with comparable sealing performance, the following must beconsidered. A lower molecular weight polymer would require a greaternumber of chemical crosslinks to reach equivalent effective crosslinkdensity; that is, network formation would be more difficult to achieve.Also, the network so formed would be expected to be "less perfect,"possibly resulting in lower tensile strength. On the other hand, apolymer exceeding the high end of the range would be more difficult toblend with the other polymers of the composition, especially the liquidtackifying component. In addition, processability through, for example,extrusion equipment, would be more difficult, resulting possibly inexcessive die swell.

The preferred number average molecular weight range for the liquidtackifying polymer is from about 1,000 to about 5,000. The lower limitis established to prevent the tackifier from migrating into other tirecomponents, a circumstance which might possibly occur in time with avery low molecular weight species. In addition, the cohesive strength ofthe sealant is desirably greater using liquid tackifiers with a numberaverage molecular weight greater than the lower limit mentioned.Although the liquid tackifier can exceed the upper molecular weight ofabout 5,000 to some degree, if too high, sealant conformability will bedeminished resulting in low adhesive strength.

As to the ratio of hydrogenated polybutadiene to liquid tackifier, thepreferred ratio is 10-30 hydrogenated polybutadiene/90-70 liquidtackifier. The lower limit of the blend ratio; i.e., 10/90 is set inorder to impart sufficient elastic properties and cohesive strength tothe sealant composition. The upper limit is established to insure thatthe composition retains the proper degree of flexibility and tack. Ifthe upper ratio is exceeded, the modulus could possibly become too highand/or the tack too low, thus resulting in poor sealant performance.

As to alternate tackifying polymers that can be utilized in the practiceof our invention, any liquid polymer which is fully compatible with thenetwork polymer can be substituted for the polybutene liquid tackifier.Among these are, for example, liquid ethylene-propylene copolymer,liquid ethylene-propylene-diene terpolymer, liquid polybutadiene, liquidhydrogenated polybutadiene and liquid butyl rubber. Known solid,hydrocarbon tackifying resins can replace from about 1-20% of the liquidtackifier.

As to alternate network polymers that can be utilized, the hydrogenatedpolybutadiene component of the present invention sealant compositionincludes the hydrogenated versions of the various isomeric forms ofpolybutadiene homopolymer such as the 1,2 and 1,4 structures, and otherhydrogenated structures derived from 1,3 butadiene. Also included arethe hydrogenated versions of butadiene copolymer such as, for example,hydrogenated, random copolymers of styrene/butadiene,acrylonitrile-butadiene, and other diene containing polymers such as,for example, EPDM; the particular structure selected is not critical.

DESCRIPTION AND FUNCTION OF COMPOUND INGREDIENTS Indopol H-300: AmocoChemicals Corp.

The backbone structure of this polymer consists essentially ofpolyisobutylene; however, a small amount of 1- and 2-butenes may bepresent in the lower molecular weight fractions. The polymer is preparedby polymerizing an isobutylene-rich stream with a metal halide catalyst.

The function of this liquid polymer in the composition is: (a) to lowerthe modulus of the network polymer thus providing the composition withthe required degree of conformability to effectively function as apuncture sealant, and (b) to increase the tack of the composition, thusincreasing its ability to adhere to puncturing objects.

Vistanex L-80: Exxon Chemical Co.

This material is a high molecular weight solid elastomer ofpolyisobutylene. It is intended to be used in the irradiation-curablesealant composition where its function is: (a) to provide sufficientgreen strength to the composition initially, thus enabling the sealantto be incorporated into the green tire via conventional tire buildingtechniques, and (b) after degrading to a lower molecular weight speciesupon irradiation, to provide the composition with increased tack.

MT Carbon Black

This material is a non-reinforcing filler that can be utilized in ourcomposition to provide increased processability and supply reactionsites during crosslinking of the network forming polymer.

The last four additives listed in Table I, page 13 of the patentapplication comprise a conventional accelerated-sulfur curing packagewith sulfur being the primary crosslinking agent. The degree ofcrosslinking achieved with this or other curing systems is such as toprevent flow of the sealant at the high temperature experienced in therunning tire and to provide the sealant with sufficient resiliency forproper sealant performance.

The term liquid tackifier, as used herein, refers to the polymer in theliquid state per se.

Fully compatible implies that the polymer components of the compositionform a homogeneous blend and will not phase-separate during the servicelife of the sealant. Insufficient compatibility, on the other hand,could result in a blend morphology wherein the network polymer, owing toits higher viscosity and low concentration, becomes the dispersed phasein an otherwise continuous liquid polymer medium. Such a morphologycould manifest itself in poor physical properties, e.g., poorelasticity, poor resistance to flow, etc., thus rendering thecomposition ineffective as a puncture sealant. As a complication, theseparated liquid polymer could possibly migrate into other tirecomponents for which it may have equal or greater affinity.

Tack in the sense used herein refers to the ability of the sealant towet the surface of and adhere to puncturing objects as well as to thebase stock of the tire.

Resilience refers to the ability of the sealant to respond quickly toand follow the displacement motions of the nail during tire travel. In astricter sense, it is a measure of the sealant's ability to recover fromdeformation.

Strength refers to the cohesive strength of the composition as itrelates to the ability to effectively seal the puncture site, after thenail is removed, without being blown through the site by the internalpressure of the tire.

Balanced properties refers to a combination of elasticity, tackiness andcohesive strength such that the composition will function effectively asa puncture sealant. That is, if the material is too elastic, forexample, its conformability would diminish, hence its ability to wet(thus adhere to) the surface of a puncturing object would diminish. Ifthe material were deficient in strength, its ability to seal a puncturesite from which the nail had been removed would be decreased as thematerial might not be able to withstand the internal pressure of thetire. Finally, if the material possessed a good measure of bothelasticity and strength but was deficient in tack, the ability to adhereto puncturing objects would be diminished.

Alternate Curing Systems

The sealant composition of the present invention can be cured with anysuitable crosslinking agent, functioning either alone or in the presenceof an activator, which can effect crosslinking to the required level.Among these are the conventional accelerated-sulfur types, quinoid typesactivated by a suitable oxidant, and the use of electron beamirradiation.

A combination of chemical and irradiation cure would be desirable, forexample, in the case where the sealant, in the form of a separate layer,is to be incorporated into the green tire. To increase the sealant'sresistance to flow during storage and tire building, the sealant layercould be given a light precure at R.T. via irradiation. Full cure wouldthen be effected chemically at the higher temperature in the tire curingpress.

Alternate Solvents

In addition to hexane, other hydrocarbon solvents capable of dissolvingthe network polymer and tackifying polymers (or resins) could be used,e.g., cyclohexane, toluene, pentane and heptane.

The following example is representative and can be varied within thecontext of our total specification disclosure as it would becomprehended and practiced by one skilled in the art. Compositioncomponents are listed on a dry weight basis.

EXAMPLE

Compositions 1 to 4 to Table I were prepared by combining the listedingredients in hexane as solvent. After blending, then respectivecompositions were stripped of solvent and then compression molded (i.e.,cured) for 30 min. at 150° C.

                  TABLE I                                                         ______________________________________                                                      1    2        3      4                                          ______________________________________                                        Hydrogenated 1,2 PBd                                                                          20     20       20   20                                       Indopol H-300   80     80       80   40                                       Vistanex L-80   --     --       --   40                                       MT Carbon Black 10     10       10   10                                       Sulfur          2      1        0.5  2                                        Methylbenzothiazyl                                                                            1.25   0.63     0.31 1.25                                     Disulfide                                                                     Diorthotolylguanidine                                                                         1.5    0.75     0.38 1.5                                      ZnO             3      1.5      0.75 3                                        ______________________________________                                    

The hydrogenated, high vinyl polybutadiene polymer used in thesecompositions was prepared from a base polymer of polybutadiene having anM_(n) =325,000, M_(w) =383,000 and a microstructure comprising, on amole basis, 98% vinyl isomer and 2% cis and trans isomer.

A diluted solution of the base polymer in toluene was charged to thereactor vessel, after which the contents were degassed under vacuum atR.T. The reactor was then pressurized with hydrogen to 25 psi and thetemperature was raised to approximately 50° C. After releasing thepressure, a nickleoctoate/triisobutylaluminum/cyclohexene catalyst wasadded. The pressure in the vessel was then increased to between 94-163psi hydrogen, after which the temperature rose to approximately 88° C.The reaction was allowed to proceed until 85.5 mole % of vinyl groupshad been hydrogenated.

The cured stocks of Table I were evaluated as regards stress-strainproperties, crosslink density and stock recovery. Stress-strain data wasobtained using 1/4" strips tested in triplicate on the Instron.Crosslink density was determined from swelling measurements in hexaneand reported as the volume fraction of rubber, V_(r), remaining in theswollen sample. Finally, stock recovery is reported as the ratio of thereturned area to the total area under the stress-strain curve determinedat 500% elongation (except for composition 4 which was tested at 300%elongation). Table II illustrates the above findings.

                  TABLE II                                                        ______________________________________                                                 300%    Tens.                                                        Composition                                                                            Mod.    Str.    Ult. Elong.                                                                           V.sub.r                                                                             Recovery                               No.      psi     psi     %       × 10.sup.2                                                                    %                                      ______________________________________                                        1        3.4     11      690     2.58  83                                     2        3.1     9.4     845     1.35  69.7                                   3        1.5     4.3     945     1.21  59.8                                    4*      16      23      480     3.91  72                                     ______________________________________                                         *Irradiated to 10 Mrads subsequent to curing.                            

A comparison of the data in Table II illustrates the dependence ofstress-strain properties and stock recovery on crosslink density andpresence of higher molecular weight tackifying polymer (Vistanex L-80).

By selecting level of network polymer, degree of crosslinking, presenceof higher molecular weight, tackifying polymer, etc., a sealantcomposition possessing an optimum combination of tackiness, resiliencyand strength can be achieved. A composition of this nature isparticularly suitable for use under dynamic conditions such as would beexperienced in a tire; however, the sealant composition of thisinvention would also find utility as, for example, a caulking compoundor roofing sealant.

It will be understood that the various composition components andprocedures of the preceding representative example can be varied withfunctionally equivalent components, in the required proportions toachieve essentially the same results with a minimum of routineexperimentation and/or optimization.

We claim:
 1. A pneumatic tire in combination with a sealant layercontaining conventional additives and, as essential rubber components, afully compatible blend of (1) a high molecular weight, hydrogenatedpolybutadiene homopolymer having a molecular weight of between 80,000and 400,000, a vinyl content prior to hydrogenation of between 10 and98% and a level of hydrogenation of between 40 and 98%, and (2) a lowmolecular weight liquid elastomer, said hydrogenated polymer providing acrosslinked network in said elastomer wherein the ratio of thepolybutadiene (1) to elastomer (2) is 10-30 to 90-70.
 2. A pneumatictire in combination with a sealant layer containing conventionaladditives and, as essential rubber components, a fully compatible blendof (1) a high molecular weight, hydrogenated polybutadiene homopolymerhaving a molecular weight of between 80,000 and 400,000, a vinyl contentprior to hydrogenation of between 10 and 98% and a level ofhydrogenation of between 40 and 98%, and (2) polybutene, saidhydrogenated polymer providing a cross-linked network in said elastomerwherein the ratio of the polybutadiene (1) to polybutene (2) is 10-30 to90-70.
 3. The tire of claim 1 wherein the polybutadiene (1) is a blendof two or more hydrogenated polybutadienes.
 4. The tire of claim 1wherein the polybutadiene (1) is a blend of a high vinyl, prior tohydrogenation, polybutadiene and a low vinyl, prior to hydrogenation,polybutadiene.
 5. The tire of claim 1 or 2 wherein the elastomer (2) hasa number average molecular weight of from about 1,000 to about 5,000. 6.The tire of claim 1 wherein the elastomer (2) is a liquid polymer ofnumber average molecular weight of from about 1,000 to about 5,000selected from the group consisting of polybutene, ethylene-propylenecopolymer, ethylene-propylenediene terpolymer, polybutadiene,hydrogenated polybutadiene and butyl rubber.
 7. The tire of claim 6wherein about 1-20% of the liquid polymer is replaced with aconventional solid, hydrocarbon tackifying resin.
 8. The tire of claim 2wherein the polybutadiene (1) is a high vinyl polybutadiene hydrogenatedto a level of about 85% having a number average molecular weight ofabout 325,000 and the ratio of polybutadiene (1) to polybutene (2) isabout 20 to about
 80. 9. The tire of claim 2 wherein the polybutadiene(1) is a high vinyl polybutadiene having a number average molecularweight of about 325,000 hydrogenated to a level of about 85 mole % ofthe vinyl groups, there is also present a high molecular weight solidelastomer of polyisobutylene and the ratio of polybutadiene (1) topolybutene (2) to solid polyisobutylene is about 20 to about 40 to about40 and the tire has been irradiated to about 10 Mrads subsequent tocuring.